Blender compositing, blur node problem
Just use a Dilate/Erode node set to 'Feather' instead of a Blur. Setting a negative Distance will erode the edges based on the set Falloff - so it will always be within the region of the mask. For more control of the falloff, set the Falloff of the Dilate/Erode to Linear and add a Color Ramp node or similar so you can control the falloff profile manually.



Note the Split Viewer showing the comparison in the Image Viewer window between original (right-hand side) and 'feathered' (left-hand side).